1. Technical Field
The present invention relates to a technique for driving a capacitive load of which a capacitive component changes or a technique for driving a plurality of capacitive loads having different capacitive components in a switching manner.
2. Related Art
There are many actuators, which operate by applying a predetermined driving signal thereto, such as an ejection head mounted in an ink jet printer and the like. In a case where the driving signal is generated by using an analog amplifier circuit, a large current flows through the circuit, whereby the power consumption is high. As a result, the power efficiency is low, and the size of a circuit substrate is large. In addition, since the consumed power is converted into heat, a large heat radiation plate is necessary, whereby the size of the substrate further increases.
Thus, a technique is proposed in which a drive waveform signal as a reference for a drive signal is converted into a modulated signal once, the acquired modulated signal is amplified, and then an amplified drive signal is acquired through a low pass filter, instead of directly amplifying an analog drive signal (JP-A-2007-168172). The amplification of the modulated signal can be realized by only switching of a switch to be turned On or Off. Furthermore, the low pass filter can be realized by using an LC circuit acquired by combining a coil and a capacitor, and accordingly, power is not consumed in principle. Thus, according to the proposed technique, the drive signal can be generated without incurring high power consumption, and as a result, not only the power efficiency is improved but also the size in the circuit substrate can be decreased. However, according to the proposed technique, since the low pass filter is configured by the LC circuit, there is a resonance characteristic in a high-frequency band, whereby it is difficult to acquire a desired drive signal. In order to suppress the resonance characteristic, there is a method in which a resistor is inserted into the low pass filter. However, according to such a method, power is consumed when a current flows through the resistor, and accordingly, an original purpose of decreasing the size in the circuit substrate by improving the power efficiency diminishes.
Thus, a technique is proposed in which a drive signal applied to an actuator is converted by using an A/D converter, and a stable drive signal is acquired by performing a stabilization process such as differential calculation as digital signal processing so as to suppress the resonance characteristic (JP-A-2010-46989). According to this technique, by configuring a state stabilizing mechanism that is used for estimating the magnitude of a current flowing through a piezoelectric device based on a digital drive signal and a digital load voltage signal, the resonance characteristic of the low pass filter can be suppressed without using the resistor.
However, according to the proposed technique, the digital signal processing of the state stabilizing mechanism is complicated, and it takes ten to several tens of clocks to complete the process, and accordingly, a delay time required for negative feedback is long. For example, in a case where a clock frequency of a digital signal processing IC is several tens of MHz, it takes several hundreds of nanoseconds to several microseconds to complete the process. Thus, in order to increase a frequency of a frequency component of the drive signal up to several hundreds of kHz, for a period component of several microseconds as a reciprocal of the frequency, a total delay time including the time required for the digital signal processing incurs a phase delay of 180 degrees or more, whereby the stability of a negative-feedback system decreases. In addition, in a case where a specified capacitive load to be driven changes, there is a problem in that the drive signal may be distorted. The reason for this is as follows. For example, in an ejection head mounted in an ink jet printer, ink is ejected by driving a piezoelectric device, and the number of piezoelectric devices simultaneously driven markedly changes depending on an image to be printed. Since the piezoelectric device is a capacitive load having a capacitive component, an increase in the number of piezoelectric devices to be driven means an increase in the capacitance of the low pass filter for generating the drive signal. Thus, when the capacitance increases, the frequency characteristics of the low pass filter change. As a result, the acquired drive signal is distorted due to the influence of the change in the frequency characteristics. In addition, in a case where a piezoelectric device is used as an actuator built in an attachment for an apparatus that is used with attachments having different characteristics being attached in a switched manner, a similar problem may occur. In other words, since the magnitude of the capacitive component of the piezoelectric device differs depending on the installed attachment, the frequency characteristics of the low pass filter changes, and accordingly the drive signal may be distorted.